1) Field of the Invention
The present invention relates to optical transmitters and optical transmission methods suitable for use in optical WDM (Wavelength-Division Multiplexing) communication systems.
2) Description of the Related Art
WDM transmission techniques, which employ 2.5-Gbps (Gbit/s) or 10-Gbps intensity modulation (on-off keying) optical signals of tens of wavelengths, have now been put to practical use in land transmission systems and submarine transmission systems, such as access network systems, metro network systems, long-distance network systems, etc. For 40-Gbps WDM systems that will be put to practical use in a short time, element techniques and development of devices have accelerated in recent years, and the 40-Gbps WDM systems are required to have the same transmission distance and frequency utilization factor as the 10-Gbps transmission systems.
As a means of realizing 40-Gbps WDM transmission systems, deep study has been devoted to modulation methods such as optical duobinary, CS-RZ (Carrier Suppressed-Return to Zero), DPSK (Differential Phase-Shift Keying), DQPSK (Differential Quadrature Phase-Shift Keying), and the like. These modulation methods are promising modulation techniques as a means of realizing 40-Gbps WDM transmission systems, because they are better in part or all of frequency utilization factor, OSNR (Optical Signal-to-Noise Ratio), and nonlinearity endurance than NRZ (Non-Return-to-Zero) used in transmission systems of 10 Gbps or less.
Among these modulation methods, DQPSK is a method in which one wavelength channel is modulated into four different phase angles to simultaneously transmit two bits per code. In this method, the pulse repeated frequency, that is, code transmission speed is reduced to half (e.g., 20 GHz) with respect to a data transmission rate (e.g., 40 Gbps). Compared with the conventional on-off keying, the signal spectral width becomes about half, so DQPSK is superior in frequency utilization factor, chromatic dispersion tolerance, and optical device transmission characteristics. Because of this, in the field of optical transmission systems, the use of phase modulation methods, such as DPSK, DQPSK, etc., is being actively investigated.
The WDM transmission systems, which have widely been put to practical use in various systems and employ 2.5-Gbps or 10-Gbps intensity modulation (on-off keying) optical signals, can be strengthened by increasing the number of wavelengths to be multiplexed. For example, some of C-band optical amplifiers have an optical signal band of about 32 nm, so if wavelength spacing is 100 GHz (about 0.8 nm), a maximum number of 40 channels can be transmitted. The WDM transmission systems themselves have the capacity of transmitting 40 channels. But, depending on the operating state of a network, users gradually increase the number of wavelengths that are used.
However, as set forth above, if the number of wavelengths multiplexed is increased to strengthen systems, the wavelength spacing becomes narrower, the walk-off quantity between wavelengths becomes smaller, and the influence of cross-phase modulation (which is nonlinear effects between wavelengths) becomes greater. In cross-phase modulation, the refractive index of an optical fiber changes in proportion to variations in the intensity of one optical channel, and this change in the refractive index modulates the phase of other optical channels.
FIGS. 42A to 42C conceptually show the phenomenon of cross-phase modulation between optical pulses. In FIG. 42A, λ1 and λ2 represent two optical pluses of different wavelengths, and from chromatic dispersion characteristics of an optical fiber, suppose that the speed of a first optical pulse of λ1 is faster than that of a second optical pulse of λ2. When the two optical pulses propagate through an optical fiber, the first optical pulse of λ1 travels faster than the second optical pulse of λ2. Therefore, as shown in FIG. 42B, the rising part of the first optical pulse of λ1 begins to overlap with the falling part of the second optical pulse of λ2.
At that stage, the falling part of the second optical pulse of λ2 is shifted in phase by red chirp caused by the rising part of the first optical pulse of λ1. If the two optical pulses of λ1 and λ2 further travel, as shown in FIG. 42C, the first optical pulse of λ1 will pass the second optical pulse of λ2 and the falling part of the first optical pulse of λ1 will overlap with the rising part of the second optical pulse of λ2. At this stage, the rising part of the second optical pulse of λ2 is shifted in phase by blue chirp caused by the falling part of the first optical pulse of λ1.
FIGS. 43A, 43B, 43C, and 43D are diagrams used to explain how chirp remains behind because of cross-phase modulation that arises during each relay span. As shown in FIG. 43A, in the case where transmission fibers 210 with a positive dispersion value are connected in multiple stages through a plurality of lumped-parameter optical amplifiers 220, as shown in FIG. 43B, an optical signal being transmitted through the transmission fiber 210 between two lumped-parameter optical amplifiers 220 has a relatively large optical power immediately after the lumped-parameter optical amplifier 220, but the optical power attenuates with propagation (first relay span). Then, if the optical signal is input to the next lumped-parameter optical amplifier 220, it is amplified to a relatively large optical power again and transmitted through the downstream transmission fiber 210. However, the optical power attenuates again with propagation (second relay span).
Now, suppose that the relay span between two lumped-parameter optical amplifiers 220 in FIG. 43A is 100 km, the chromatic dispersion near the wavelength of an optical signal is 2.5 ps/km/nm, the signal wavelength spacing is 0.8 nm, and the data transmission rate is 20 Gbit/s. The delay of an optical signal of an adjacent wavelength caused by chromatic dispersion can be expressed by the following Equation:Delay(ps)=D(ps/km/nm)×L(km)×Δλ(nm)  (1)where
D=chromatic dispersion,
L=transmission distance,
Δλ=wavelength spacing.
If a wavelength multiplexed optical signal is transmitted 25 km, the time delay between adjacent wavelengths is 2.5 (ps/km/nm)×25 (km)×0.8 (nm)=50 (ps) from Equation (1). This means that an adjacent optical pulse delays 1 bit because the bit interval of a 20-Gbit/s signal is 50 ps.
That is, at the spot (coordinates 0 km) immediately after a lumped-parameter optical amplifier 220, in the case where optical pulses of adjacent wavelengths λ11 and λ2 are about to, what is called, collide with each other (see FIGS. 42A to 42C), red chirp and blue chirp arise once during a transmission of 25 km. Therefore, in the case of a relay span of 100 km, the time delay is 2.5 (ps/km/nm)×100(km)×0.8(nm)=200 (ps). That is, the time delay between optical pulses of adjacent wavelengths is four bits, so blue chirp arises 4 times and red chirp arises 4 times. For instance, if the second optical pulse of λ2 passes through a transmission fiber 210 with a length of 100 km, it will arrive after a delay of four bits from the first optical pulse of λ1.
Now, assuming the signal bit patterns of two different adjacent wavelength signals λ1 and λ2 are all “1”s, consider the cross-phase modulation between the two wavelengths λ1 and λ2. After a wavelength multiplexed signal has been amplified to a desired level by the lumped-parameter optical amplifier 220, it is transmitted through the transmission fiber 210 of the next stage. During the span of this transmission fiber 210, the above-described cross-phase modulation arises between the two different wavelengths λ1 and λ2.
When there is no optical power attenuation over the span of the transmission fiber 210 through which the two different wavelengths λ1 and λ2 are transmitted, the quantities of red chirp and blue chirp that the second optical pulse of λ2 undergoes by the first optical pulse of λ1 become equal to each other. Therefore, they cancel each other when the first optical pulse of λ1 passes the second optical pulse of λ2. As a result, there is no possibility that the second optical pulse of λ2 will be shifted in phase by cross-phase modulation.
However, as shown in FIG. 43B, the optical power of an optical signal is attenuated as it is propagated through the transmission fiber 210. Therefore, as shown in FIG. 34C, the quantity of red chirp, which arises as first optical pulse of λ1 overlaps with the second optical pulse of λ2, is slightly larger than that of blue chirp that arises as the first optical pulse of λ1 passes the second optical pulse of λ2 from the overlapping state. Because of this, the two chirp quantities will not cancel each other, and the red chirp that has arisen at the location of the higher optical power will remain behind slightly.
Thus, during the span of the transmission fiber 210, as shown in FIG. 43D, red chirp always remains behind because of cross-phase modulation, and in total, it remains behind by ΔC. In this case, the second optical pulse of λ2 is shifted in phase by the residual red chirp, and the group speed of a wavelength multiplexed signal passing through an optical fiber changes.
In a relay transmitter including the lumped-parameter optical amplifier 220, dispersion compensation is normally performed to suppress waveform degradation. In the case where the propagation time delay difference between adjacent wavelengths λ1 and λ2 is compensated at each relay stage with the dispersion compensating function, and the bit patterns of adjacent wavelength signals input to the lumped-parameter amplifier 220 of each stage become practically the same, the residual red chirp in each relay stage accumulates. Because of this, as shown in FIGS. 43C and 43D, the residual chirp ΔC during each relay span is multiplied by the number of relay stages, resulting in severe waveform degradation.
Therefore, the optical pulse of λ2 is shifted in phase by the residual red chirp, the group speed passing through an optical fiber changes, and this influence remains behind at the end of each relay span. In the case of intensity modulation optical signals, the residual chirp causes jitter (deviation from the center position of a received pulse) at the signal receiving end, and in the case of phase modulation optical signals, the residual chirp becomes the direct noise component of a symbol code and degrades transmission performance.
Thus, because intensity distortion between adjacent wavelengths causes phase fluctuations and degrades transmission properties, cross-phase modulation has become an important consideration in wavelength multiplexing long-distance transmission systems in which a collision of optical pulses is repeated between adjacent wavelengths.
Note that the related art of the present invention is shown in non-patent document 1 (G. Charlet, et. al., “Nonlinear Interactions Between 10 Gb/s NRZ Channels and 40 Gb/s Channels with RZ-DQPSK or PSBT Format, over Low-Dispersion Fiber”, Mo3.2.6, ECOC 2006).
Multiplexing and transmitting channels of phase modulation optical signals (modulated in phase modulation methods described above) and channels of intensity modulation optical signals are being examined. That is, multiplexing intensity modulation and phase modulation optical signals can increase the frequency utilization factor, and employ the existing equipment for WDM transmission of intensity modulation optical signals to upgrade transmission systems in steps and efficiently.
However, in upgrading transmission systems, when it is necessary to multiplex and transmit intensity modulation and phase modulation optical signals, nonlinear effects (cross-phase modulation) due to variations in the intensity of an intensity modulation optical signal greatly affect the reception quality of phase modulation optical signals.
That is, in systems for WDM transmission of intensity modulation optical signals, for example, nonzero dispersion-shifted fibers (NZDSFs) with a relatively small chromatic dispersion value per unit of length are sometime employed as transmission fibers to balance self-phase modulation and cross-phase modulation so that optimum reception signal quality is obtained.
However, in transmission systems with transmission fibers whose chromatic dispersion is small per unit of length, if the function of multiplexing phase modulation optical signals such as DQPSK signals is added, in nonzero dispersion-shifted fibers it cannot prove definitely that the code walk-off (or relative propagation speed difference in a transmission fiber) between wavelength channels is great enough to avoid the influence of cross-phase modulation that phase modulation optical signals undergo.
In other words, when employing nonzero dispersion-shifted fibers (NZDSFs), if phase modulation optical signals are arranged according to the conventional channel arrangement that could be used for wavelength-multiplexing intensity modulation optical signals, and are multiplexed along with phase modulation optical signals of other channels, the degradation of the reception signal quality of phase modulation optical signals due to cross-phase modulation becomes greater compared with the case of WDM transmission of only phase modulation optical signals.
The aforementioned non-patent document 1 has attained the following results. That is, in the case where 10-Gbps intensity modulation optical signals and 43-Gbps phase modulation optical signals are multiplexed and transmitted through a nonzero dispersion-shifted fiber, if the 10-Gbps intensity modulation optical signals are arranged at wavelengths adjacent to the 43-Gbps phase modulation optical signal, the reception signal quality is degraded compared with the case where 43-Gbps phase modulation optical signals are arranged at all wavelength channels. The non-patent document 1 has also attained the following results. That is, even when the state of polarization is optimum (orthogonal), a Q value representing reception signal quality degrades compared with WDM transmission of only 40-Gbps phase modulation optical signals, and in a parallel polarization state, the Q value degrades about 3 dB.
That is, in upgrading transmission systems, in the case of multiplexing and transmitting intensity modulation and phase modulation optical signals, it is necessary to take more effective countermeasures to suppress cross-phase modulation, compared with the case of the conventional WDM transmission of only 10-Gbps intensity modulation optical signals or WDM transmission for phase modulation optical signals.
The non-patent document 1 does not teach or suggest a means of suppressing the influence of cross-phase modulation that phase modulation optical signals undergo when multiplexing and transmitting intensity modulation and phase modulation optical signals.